CD19 is a type I membrane protein expressed by all B cells and B cell precursors but not by plasma cells or other cell types. CD19 functions as part of the B cell antigen receptor complex, allowing B cells to respond to foreign antigens. CD19 is also expressed on all B cell malignancies and is a favorable target for immunotherapy because of its specificity; patients receive immunoglobulin injections because their own B cells are depleted along with the malignant cells.